[CanvasKit] Add workaround for unimplemented computeLineMetrics (#441)

Related to #437

Author: daohoangson

.github/workflows/demo_app.yml

@@ -51,7 +51,15 @@ jobs:
           pwd && ls -al
           sed -i -e 's/# web only: //' pubspec.yaml
           
-          flutter build web
+          flutter build web --web-renderer canvaskit
+          mv ./build/web ./build/web-canvaskit
+
+          flutter build web --web-renderer html
+          mv ./build/web ./build/web-html
+
+          flutter build web --web-renderer auto
+          mv ./build/web-canvaskit ./build/web/canvaskit
+          mv ./build/web-html ./build/web/html
 
       - name: Generate supported/tags.html
         run: |
@@ -94,6 +102,6 @@ jobs:
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
         with:
-          body: "Deployed ${{ github.sha }} to ${{ steps.now.outputs.url }}.\n\nQuick links:\n- [supported/tags.html](${{ steps.now.outputs.url }}/supported/tags.html)\n- [Hello World](${{ steps.now.outputs.url }}/#/helloworld)\n- [Hello World (core)](${{ steps.now.outputs.url }}/#/helloworldcore)\n- [Wordpress](${{ steps.now.outputs.url }}/#/wordpress)"
+          body: "Deployed ${{ github.sha }} to ${{ steps.now.outputs.url }}.\n\nQuick links:\n- [supported/tags.html](${{ steps.now.outputs.url }}/supported/tags.html)\n- [Hello World](${{ steps.now.outputs.url }}/#/helloworld)\n- [Hello World (core)](${{ steps.now.outputs.url }}/#/helloworldcore)\n- [Wordpress](${{ steps.now.outputs.url }}/#/wordpress)\n- [--web-renderer canvaskit](${{ steps.now.outputs.url }}/canvaskit/)\n- [--web-renderer html](${{ steps.now.outputs.url }}/html/)"
           fingerprint: "## Demo app"
           replace: please

packages/core/lib/src/internal/core_ops.dart

@@ -1,4 +1,5 @@
 import 'dart:math';
+import 'dart:ui';
 
 import 'package:flutter/gestures.dart';
 import 'package:flutter/rendering.dart';

packages/core/lib/src/internal/ops/tag_li.dart

@@ -372,11 +372,17 @@ class _ListMarkerRenderObject extends RenderBox {
   @override
   void paint(PaintingContext context, Offset offset) {
     final canvas = context.canvas;
-    final m = _textPainter.computeLineMetrics().first;
+
+    var lineMetrics = <LineMetrics>[];
+    try {
+      lineMetrics = _textPainter.computeLineMetrics();
+      // ignore: empty_catches
+    } on UnimplementedError {}
+    final m = lineMetrics.isNotEmpty ? lineMetrics.first : null;
     final center = offset +
         Offset(
           size.width / 2,
-          (m.descent.isFinite && m.unscaledAscent.isFinite)
+          (m?.descent?.isFinite == true && m?.unscaledAscent?.isFinite == true)
               ? size.height -
                   m.descent -
                   m.unscaledAscent +